Sub-Terahertz Spin Relaxation Dynamics of Boron-Vacancy Centers in Hexagonal Boron Nitride

Abstract

Quantum sensors based on spin-defect relaxation have become powerful tools for detecting faint magnetic signals, yet their operation has remained largely confined to low magnetic fields and gigahertz frequencies. Extending such sensors into high-field ($>0.3$ T) and sub-terahertz regimes would enable quantum metrology across a wide range of electromagnetic phenomena and scientific applications, but has proven challenging. Here, we demonstrate that negatively charged boron vacancies (${\mathrm{V}}_{\mathrm{B}}^{-}$) in two-dimensional hexagonal boron nitride can function as relaxation-based quantum sensors operating up to 0.2 terahertz. Their uniform spin-orientation and persistent spin-contrast at high fields enable direct measurement of intrinsic spin relaxation across previously unexplored temperature and frequency regimes. We also reveal a crossover in relaxation behavior \textemdash initially decreasing at low fields before rising at higher fields \textemdash consistent with the emergence of single-phonon-induced resonant noise that becomes significant at sub-terahertz frequencies. These results establish ${\mathrm{V}}_{\mathrm{B}}^{-}$ centers as a versatile platform for quantum sensing in the sub-terahertz, high-field regime.